1. Field of the Invention
This invention relates generally to a fuel and air distribution system for use in a gas turbine combustor for reducing the production of nitrogen oxide and other air pollutants and specifically to a premix fuel nozzle for use in the combustion chambers for gas turbine combustors.
2. Description of Related Art
The U.S. Government has enacted requirements for lowering pollution emissions, and in particular, for lowering the amounts of nitrogen oxide (NOx) and carbon monoxide produced by natural gas powered turbines, which in turn generate electricity by being connected to electrical generators. The combustion process of air and natural gas as a fuel in a gas turbine combustion chamber produces pollutants such as nitrogen oxides, which are both NO and NO2, designated generally as NOx. U.S. Government has enacted requirements for lowering pollution emissions, especially for lowering the NOx produced by gas turbines during power generation. Complying with these regulations can be difficult utilizing conventional diffusion burners where the natural gas fuel is introduced directly via a fuel nozzle into the combustion chamber where it is mixed with combustion air. U.S. Pat. No. 4,589,260, issued to Krockow, May 20, 1986, describes a premixing burner with integrated diffusion burner to reduce NOx pollutants in the combustion within a gas turbine combustion chamber. The Krockow invention discloses a burner system for gas turbine combustion chambers which is comprised of using a fuel/air premixing burner with an integrated diffusion burner. The premixing burner has a premixing chamber. The diffusion burner has a main fuel nozzle which is arranged in the central zone of the flame retention baffle. In particular, the premix nozzle includes a series of radial arms disposed into the air flow for premixing the fuel and air prior to its combustion downstream of the diffusion nozzle. One of the problems with such a hybrid system is that the premix nozzles, being on radial arms or pegs, actually do not provide for a uniform distribution of air and fuel mixture and can act as a flame holder. The peg design also produces unmixed areas of fuel and air flow downstream of the pegs.
In response to the requirement of lower pollution with NOx, the industry has adopted a dual-stage, dual-mode, low NOx combustor for use in gas turbine engines. Each of these combustors comprises a primary combustion chamber and a secondary combustion chamber separated by a venturi throat region. The primary combustion chamber includes primary fuel nozzles that deliver fuel into the primary combustion chamber. In a typical system, there are a plurality of primary nozzles arranged in an annular array around a secondary nozzle. For example, each combustor may include six primary fuel nozzles and one secondary fuel nozzle centrally located relative to the six fuel nozzle array. Fuel, which is typically natural gas (but could be any suitable liquid fuel or gaseous fuel), is delivered to each of the primary nozzles by an appropriate fuel pipe. Ignition in the primary combustor takes place by the use of spark plugs within the primary nozzle region.
Surrounded by a plurality of fuel nozzles is an elongated secondary nozzle which is situated somewhat downstream of the primary nozzles. The purpose of the secondary nozzle is to alternately maintain a pilot flame so that the combustion continues in the secondary combustion chamber once the primary chambers' flames have been extinguished. U.S. Pat. No. 4,982,570, issued to Waslo, et al., describes a premix pilot nozzle for dry low NOx combustors that utilizes an integrated, combined, premix nozzle and diffusion nozzle similar to that disclosed in U.S. Pat. No. 4,589,260 to Krockow. The premix nozzle in the '570 Patent is also a plurality of radial fuel distribution tubes which extend radially outward from the axial diffusion nozzle pipe. Each of the radial pipes include a plurality of fuel discharge holes which are directed downstream toward the discharge end of the combined diffusion nozzle. Again, such an integrated system does not provide for complete uniform premixing of air and fuel because of the structural layout of the peg-like fuel distribution arms which are integrated into the diffusion nozzle system. The air gaps get larger radially outwardly from the diffusion nozzle housing. This is especially important when the fuel nozzle system is used in the two-stage, two-mode gas turbine which includes a combustor having a primary combustion chamber and a secondary combustion chamber.
In a typical operational cycle of a two-stage, two-mode gas turbine, fuel is delivered to the primary nozzles with air flow, which is ignited by spark plugs, causing ignition and fire in the primary combustion chambers. This allows for an initial start-up of the turbine to a certain power level. At a desired turbine power level, fuel is then delivered to a secondary nozzle which is ignited from the primary combustion fires causing a pilot flame in the secondary nozzle. Transfer fuel is also provided to the secondary nozzle to increase the secondary nozzle combustion output beyond a pilot flame to allow shutdown of the primary nozzles during combustion transition between the primary combustion chambers and the secondary combustion chambers. Once the secondary pilot flame has been established and transfer fuel is flowing, fuel is shut off to the primary nozzle causing a flame-out in the primary combustion chambers. After flame-out, the fuel supply is again turned on to the primary nozzles and mixed with air. The primary fuel/air mixture flows from the primary combustion chambers into the combustor's secondary combustion chamber past the venturi passage and is continuously ignited by the fire in the secondary combustion chamber. Transfer fuel is shut off in the secondary nozzle. The pilot light in the secondary nozzle is thus used to maintain and insure continuous combustion in the secondary combustion chamber at all times.
The secondary nozzle has also been found to contribute to NOx pollution, especially when functioning as a diffusion nozzle.
Although secondary fuel nozzles that have an integrated premix nozzle and diffusion nozzle pilot light have improved combustion, reducing pollutants, any improvement in further reducing NOx and CO pollutants is important.
The present invention provides for a nozzle system that has a diffusion nozzle for maintaining the pilot light and providing transfer fuel and a separate premix annular full ring fuel distributor separated away from the diffusion nozzle structure and surrounding the diffusion nozzle structure in such a way as to increase the thorough mixing of fuel and air in a premix area resulting in higher efficiency and lower pollutants from the secondary nozzle system. The premix annular ring fuel distributor has a plurality of apertures facing downstream for discharging natural gas (or any suitable fuel), while an air stream flows completely around the surface of the annular ring, greatly enhancing the premixing of the natural gas with the air flowing around the ring.
The separate diffusion nozzle for providing transfer fuel includes a plurality of individual fuel-carrying transfer fuel tubes mounted around a plurality of air-flow channels, all of which terminates at the end of the diffusion nozzle.